Electrolyte for lithium secondary battery and lithium secondary battery comprising the same

ABSTRACT

The present disclosure relates to an electrolyte for a lithium secondary battery, comprising a non-aqueous solvent, a lithium salt and an additive having a perfluoroalkyl group. By including the additive having a specific structure in the electrolyte, the output of the lithium secondary battery can be improved greatly.

TECHNICAL FIELD

The present disclosure relates to an electrolyte for a lithium secondarybattery. More particularly, the present disclosure relates to anelectrolyte comprising a compound having a perfluoroalkyl group as anadditive and a lithium secondary battery comprising the electrolyte.

BACKGROUND ART

With the recent advancement in information technology, electronicdevices are becoming smaller sized, lighter and more portable. As aresult, demand on batteries with higher energy density as power sourceof these electronic devices is also increasing. The lithium secondarybattery is a battery capable of satisfying this requirement and studiesare being actively carried out thereabout. The lithium secondary batterycomprises a cathode, an anode, an electrolyte providing a path forlithium ions between the cathode and the anode and a separator. Electricenergy is produced through oxidation-reduction reactions as the lithiumions are intercalated into and deintercalated from the cathode and theanode.

A non-aqueous electrolyte used in a lithium secondary battery generallyincludes an electrolyte solvent and an electrolyte salt. However, theelectrolyte solvent is decomposed on the electrode surface orco-intercalated between the carbonaceous anode layers during chargingand discharging of the battery, thereby collapsing the anode structureand damaging stability of the battery.

It is known that such problems can be solved by a solid electrolyteinterface (SEI) film formed on the anode surface owing to reduction ofthe electrolyte solvent during initial charging of the battery. However,the SEI film is generally insufficient to serve as a film forcontinuously protecting the anode and its life and performance aredeteriorated after repeated charging and discharging of the battery. Inparticular, the SEI film of the existing lithium secondary battery isthermally unstable. Thus, if the battery is operated or kept at hightemperature, the SEI film may collapse easily with time due toelectrochemical energy and thermal energy. As a result, the batteryperformance is deteriorated at high temperature. Particularly, gas suchas CO₂ is continuously generated due to collapse of the SEI film,decomposition of the electrolyte, or the like. Consequently, theinternal pressure and thickness of the battery are increased.

In order to solve these problems, a method of using, for example,vinylene carbonate as an electrolyte additive for forming a film on theanode surface was proposed. Although vinylene carbonate exhibitssuperior storage performance at high temperature and superior cycleperformance, output performance is not at low temperature.

Recently, the lithium secondary battery is used for various purposes,from general electronic devices to various applications includingelectric vehicles. In this regard, demand on a high-output battery isalso increasing. Accordingly, a battery capable of providing high outputnot only at high temperature but also at low temperature is necessary.

DISCLOSURE Technical Problem

The present disclosure provides a non-aqueous electrolyte for a lithiumsecondary battery capable of improving the output characteristics of alithium secondary battery and a lithium secondary battery comprising thesame.

Technical Solution

In one general aspect, the present disclosure provides an electrolytefor a lithium secondary battery, comprising: a non-aqueous solvent; alithium salt; and a compound of a specific structure having aperfluoroalkyl group as an additive.

In another general aspect, the present disclosure provides a lithiumsecondary battery comprising the electrolyte.

Advantageous Effects

A lithium secondary battery prepared using the electrolyte of thepresent disclosure may have improved output characteristics.

In particular, a battery prepared using an electrolyte including theadditive of a specific structure described in the present disclosure hasremarkably improved output characteristics as compared to one preparedusing an electrolyte not including the additive.

DESCRIPTION OF DRAWINGS

FIGS. 1-4 show the result of an HPPC test of secondary batteries usingelectrolytes of Examples and Comparative Examples.

FIG. 1 shows HPPC output data for charging at −30° C.

FIG. 2 shows HPPC output data for discharging at −30° C.

FIG. 3 shows HPPC output data for charging at room temperature.

FIG. 4 shows HPPC output data for discharging at room temperature.

-▪-: COMPARATIVE EXAMPLE 2 --: EXAMPLE 4 -▴-: EXAMPLE 5 -▾-: EXAMPLE 6BEST MODE

The present disclosure provides an electrolyte for a lithium secondarybattery, comprising:

a non-aqueous solvent;

a lithium salt; and

one or more additive selected from compounds of Chemical Formulas 1-3.

wherein,

R₁ represents a C₂-C₈ perfluoroalkyl group;

R₂, R₃ and R₄ independently represent a C₁-C₃ alkyl group; and arepresents an integer selected from 1 to 5.

wherein,

R₆ represents a C₂-C₈ perfluoroalkyl group;

c represents an integer selected from 1 to 7; and

d represents an integer selected from 0 to 3.

wherein,

R₅ represents a C₂-C₈ perfluoroalkyl group; and

b represents an integer selected from 0 to 3.

The compound of Chemical Formula 1 may be a compound of Chemical Formula1a:

The compound of Chemical Formula 2 may be a compound of Chemical Formula2a:

The compound of Chemical Formula 3 may be a compound of Chemical Formula3a:

The compounds of Chemical Formulas 1-3 may be used either alone or incombination.

The one or more additive selected from the compounds of ChemicalFormulas 1-3 may be included in an amount of 0.1-10 parts by weightbased on 100 parts by weight of the electrolyte.

The non-aqueous solvent used in the electrolyte of the presentdisclosure may be one commonly used in an electrolyte for a lithiumsecondary battery, without particular limitation. Specifically, one ormore solvent selected from the group consisting of ethylene carbonate(EC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), dimethylcarbonate (DMC), propylene carbonate (PC), dipropyl carbonate (DPC),butylene carbonate, methyl propyl carbonate, ethyl propyl carbonate,dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane,tetrahydrofuran, N-methyl-2-pyrrolidone (NMP), γ-butyrolactone andsulfolane may be used.

The lithium salt used in the electrolyte of the present disclosure maybe one commonly used in an electrolyte for a lithium secondary battery,without particular limitation. Specifically, one or more solventselected from the group consisting of LiPF₆, LiBF₄, LiCl, LiBr, LiI,LiClO₄, LiB₁₀Cl₁₀, LiCF₃SO₃, LiCF₃CO₂, LiAsF₆, LiSbF₆, LiAlCl₄,CH₃SO₃Li, CF₃SO₃Li and (CF₃SO₂)₂NLi may be used.

The present disclosure also provides a lithium secondary batterycomprising a cathode, an anode and the electrolyte described above.

The lithium secondary battery of the present disclosure may be preparedby injecting the electrolyte into an electrode structure consisting of acathode, an anode and a separator interposed between the cathode and theanode. The cathode, anode and separator of the electrode structure maybe those commonly used in a lithium secondary battery.

As a specific example, a cathode active material used in the cathode maybe a lithium-containing transition metal oxide. For example, one or moreselected from the group consisting of LiCoO₂, LiNiO₂, LiMnO₂, LiMn₂O₄,Li(Ni_(a)Co_(b)Mn_(c))O₂ (0<a<1, 0<b<1, 0<c<1, a+b+c=1),LiNi_(1-y)Co_(y)O₂, LiCo_(1-y)Mn_(y)O₂, LiNi_(1-y)Mn_(y)O₂ (0≦y<1),Li(Ni_(a)Co_(b)Mn_(c))O₄ (0<a<2, 0<b<2, 0<c<2, a+b+c=2),LiMn_(2-z)Ni_(2-z)O₄, LiMn_(2-z)Co_(z)O₄ (0<z<2), LiCoPO₄ and LiFePO₄may be used. In addition to the oxide, a sulfide, a selenide, a halide,etc. may also be used.

As an anode active material used in the anode, a carbon material,lithium metal, silicon, tin, LTO, etc., which can intercalate ordeintercalate lithium ions, may be used in general. Specifically, acarbon material may be used. The carbon material may be low-crystallinecarbon or high-crystalline carbon. The low-crystalline carbon may besoft carbon or hard carbon as representative examples and thehigh-crystalline carbon may be natural graphite, kish graphite,pyrolytic carbon, mesophase pitch-based carbon fiber, mesocarbonmicrobead, mesophase pitch or high-temperature sintered carbon such aspetroleum- or coal tar pitch-derived coke, as representative examples.The anode may contain a binder. Various kinds of binder polymers such aspoly(vinylidenefluoride-co-hexafluoropropylene) (PVdF-co-HFP) copolymer,polyvinylidene fluoride (PVdF), polyacrylonitrile, poly(methylmethacrylate), etc. may be used as the binder.

And, the separator may be formed of a commonly used porous polymer film,for example, a porous polymer film prepared from a polyolefin-basedpolymer such as ethylene homopolymer, propylene homopolymer,ethylene-butene copolymer, ethylene-hexene copolymer orethylene-methacrylate copolymer, in a single layer or laminate form.Alternatively, the separator may be formed of a commonly used porousnonwoven fabric such as a nonwoven fabric made of high-melting pointglass fiber, polyethylene terephthalate fiber, etc., but is not limitedthereto.

The lithium secondary battery of the present disclosure may have variousshapes which are not specially limited. For example, it may have acylindrical can shape, an angled shape, a pouch shape or a coin shape.

Hereinafter, the present disclosure will be described in detail throughexamples. However, the following examples are for illustrative purposesonly and it will be apparent to those of ordinary skill in the art thatthe scope of the present disclosure is not limited by the examples.

EXAMPLES 1-3 AND COMPARATIVE EXAMPLE 1 Preparation of Electrolyte

A solvent, a lithium salt and an additive were mixed as described inTable 1 to prepare electrolytes of the present disclosure (Examples 1-3)and one for comparison (Comparative Example 1).

TABLE 1 Lithium salt Solvent (weight Additive (addition ratio based on(addition amount) amount) 100 parts by weight) Example 1 ChemicalFormula 1a LiPF₆ EC/PC/EMC (1 parts by weight) (1M) (30/20/50) Example 2Chemical Formula 2a LiPF₆ EC/PC/EMC (1 parts by weight) (1M) (30/20/50)Example 3 Chemical Formula 3a LiPF₆ EC/PC/EMC (1 parts by weight) (1M)(30/20/50) Comparative — LiPF₆ EC/PC/EMC Example 1 (1M) (30/20/50)

EXAMPLES 4-6 AND COMPARATIVE EXAMPLE 2 Preparation of Lithium SecondaryBattery

(Preparation of Cathode)

A cathode mixture slurry was prepared by adding 94 wt % of lithiumcobalt oxide as a cathode active material, 3 wt % of carbon black as aconducting agent and 3 wt % of PVdF as a binder toN-methyl-2-pyrrolidone (NMP) as a solvent. The cathode mixture slurrywas coated on a 20-μm thick aluminum (Al) foil as a cathode currentcollector and then dried to prepare a cathode.

(Preparation of Anode)

An anode mixture slurry was prepared by adding 96 wt % of carbon powderas an anode active material, 3 wt % of PVdF as a binder and 1 wt % ofcarbon black as a conducting agent to N-methyl-2-pyrrolidone (NMP) as asolvent. The anode mixture slurry was coated on a 10-μm thick copper(Cu) foil as an anode current collector and then dried to prepare ananode.

(Assemblage of Battery)

The cathode and the anode prepared above and a separator consisting ofthree layers of polypropylene/polyethylene/polypropylene (PP/PE/PP) wereassembled by stacking and then the electrolyte of Examples 1-3 andComparative Example 1 was injected respectively to prepare a battery.

The batteries of Examples 4-6 and Comparative Example 2 were prepared byinjecting the electrolytes of Examples 1-3 and Comparative Example 1,respectively.

Test Example: HPPC Test

HPPC test was performed for the batteries of Examples 4-6 andComparative Example 2.

The HPPC test is an internationally standardized method and specified bythe US Department of Energy (DOE) (FreedomCAR Battery Test Manual forPower-Assist Hybrid Electric Vehicles, DOE/ID-11069, 2003).

The HPPC test result is shown in FIGS. 1-4. FIG. 1 shows HPPC outputdata for charging at −30° C., FIG. 2 shows HPPC output data fordischarging at −30° C., FIG. 3 shows HPPC output data for charging atroom temperature and FIG. 4 shows HPPC output data for discharging atroom temperature.

As can be seen from FIGS. 1-4, the batteries prepared according to thepresent disclosure (Examples 4-6) exhibit improved outputcharacteristics as compared to the battery of Comparative Example 2. Inparticular, they exhibit improvement in output characteristics at lowtemperature.

1. An electrolyte for a lithium secondary battery, comprising: anon-aqueous solvent; a lithium salt; and one or more additive selectedfrom the group consisting of compounds of Chemical Formulas 2 and 3:

wherein R₆ represents a C₂-C₈ perfluoroalkyl group; c represents aninteger selected from 1 to 7; and d represents an integer selected from0 to
 3.

wherein R₅ represents a C₂-C₈ perfluoroalkyl group; and b represents aninteger selected from 0 to
 3. 2. The electrolyte for a lithium secondarybattery of claim 1, which further comprises a compound of ChemicalFormula 1 as the additive:

wherein R₁ represents a C₂-C₈ perfluoroalkyl group; R₂, R₃ and R₄independently represent a C₁-C₃ alkyl group; and a represents an integerselected from 1 to
 5. 3. The electrolyte for a lithium secondary batteryof claim 2, wherein the compound of Chemical Formula 1 is represented byChemical Formula 1a:


4. The electrolyte for a lithium secondary battery of claim 1, whereinthe compound of Chemical Formula 2 is represented by Chemical Formula2a:


5. The electrolyte for a lithium secondary battery of claim 1, whereinthe compound of Chemical Formula 3 is represented by Chemical Formula3a:


6. The electrolyte for a lithium secondary battery of claim 1, whereinthe additive is included in an amount of 0.1-10 parts by weight based on100 parts by weight of the electrolyte.
 7. The electrolyte for a lithiumsecondary battery of claim 2, wherein the additive is included in anamount of 0.1-10 parts by weight based on 100 parts by weight of theelectrolyte.
 8. The electrolyte for a lithium secondary battery of claim1, wherein the non-aqueous solvent is one or more selected from thegroup consisting of ethylene carbonate (EC), diethyl carbonate (DEC),ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), propylenecarbonate (PC), dipropyl carbonate (DPC), butylene carbonate, methylpropyl carbonate, ethyl propyl carbonate, dimethyl sulfoxide,acetonitrile, dimethoxyethane, diethoxyethane, tetrahydrofuran,N-methyl-2-pyrrolidone (NMP), γ-butyrolactone and sulfolane.
 9. Theelectrolyte for a lithium secondary battery of claim 2, wherein thenon-aqueous solvent is one or more selected from the group consisting ofethylene carbonate (EC), diethyl carbonate (DEC), ethyl methyl carbonate(EMC), dimethyl carbonate (DMC), propylene carbonate (PC), dipropylcarbonate (DPC), butylene carbonate, methyl propyl carbonate, ethylpropyl carbonate, dimethyl sulfoxide, acetonitrile, dimethoxyethane,diethoxyethane, tetrahydrofuran, N-methyl-2-pyrrolidone (NMP),y-butyrolactone and sulfolane.
 10. The electrolyte for a lithiumsecondary battery of claim 1, wherein the lithium salt is one or moreselected from the group consisting of LiPF₆, LiBF₄, LiCl, LiBr, LiI,LiClO₄, LiB₁₀Cl₁₀, LiCF₃SO₃, LiCF₃CO₂, LiAsF₆, LiSbF₆, LiAlCl₄,CH₃SO₃Li, CF₃SO₃Li and (CF₃SO₂)₂NLi.
 11. The electrolyte for a lithiumsecondary battery of claim 2, wherein the lithium salt is one or moreselected from the group consisting of LiPF₆, LiBF₄, LiCl, LiBr, LiI,LiClO₄, LiB₁₀Cl₁₀, LiCF₃SO₃, LiCF₃CO₂, LiAsF₆, LiSbF₆, LiAlCl₄,CH₃SO₃Li, CF₃SO₃Li and (CF₃SO₂)₂NLi.
 12. A lithium secondary batterycomprising a cathode, an anode and the electrolyte of claim
 1. 13. Alithium secondary battery comprising a cathode, an anode and theelectrolyte of claim 2.